What to do?

Okay, so my boyfriend and I typically use his old iPhone to play music for our son to help him fall asleep. For some reason calls still comes through on this old phone. So today I see he got a call from his ex. I know they’re still cordial or whatever, but out of curiosity I looked at his call log to see how often they chat. In the last 2 weeks they’ve talked twice, both 2-5mins but they occurred when he was away from home. I just don’t understand why, especially since he has no issues talking to his other friends while in my presence. I can’t say that he’s cheating, but what would your next step be in this situation? Should I try to see their texts or just bring this situation up to him and talk about it

Update: So I ended up snooping when he got home from work.. I personally know they’ve texted within the last month, but now the latest text between them are in February. At this point I’m pretty sure he’s hiding something, but I’m gonna wait until I can get solid evidence before I bring it up.